I like the color combo on the 2 white IS. To me it looks better when the section above the exhaust tips are color matched with only the middle part painted black. It gives the bumper a better flow and you can see the lines of the diffuser a lot better.

The pic of the white IS's two toned looks nasty. I'd go with that look for sure.
I'd love to have a diffuser on my car. I scrape my exhaust though so I'm pretty sure a diffuser will just make things worse for me